The present invention relates to a process for producing a ceramic structure, particularly to a process for producing a ceramic structure, which is suitable for production of a honeycomb ceramic structure using a continuous extruder.
For production of a ceramic structure, there has been widely used a process which comprises mixing a granular ceramic raw material, a dispersing agent such as water or the like, and an additive such as binder or the like, kneading the resulting ceramic raw material mixture, and extruding the resulting readily formable bullet from the die of an extruder. In particular, a process which comprises feeding a ceramic raw material mixture into a twin screw extruder, conducting therein the transfer and kneading of the mixture simultaneously, and extruding the kneaded material from the die of the extruder, is expected as a process of high productivity.
In such a production process, however, the ceramic raw material mixture having a relatively high viscosity undergoes a large amount of a mechanical energy from the equipment and consequently a large amount of heat is generated in the raw material mixture; as a result, the ceramic raw material mixture hardens owing to, for example, the gelling of the binder, resulting in forced stoppage of the equipment in some cases.
Meanwhile, in JP-B-6-35126 was proposed a production process using a twin screw continuous extruder, wherein a gel type binder having a predetermined viscosity is used and thereby kneading at higher temperatures is made possible.
In this process, however, the pressure generated in a ceramic raw material is not reduced; therefore, the load acting on the extruder is still large, the kneading blade, die, etc. of the extruder tend to undergo wear, damage, etc., and an increase in power consumption is incurred.
Further in this process, extrusion is conducted at a higher viscosity at a higher temperature; therefore, the influence of the above-mentioned heat generation in ceramic raw material is not completely removed.
Meanwhile, in JP-A-05-262558, etc. were proposed processes for production of a ceramic structure, which use a raw material batch obtained by adding, to a ceramic raw material mixture, an organic assistant such as fatty acid ester or the like and thereby enables low-pressure extrusion of readily formable bullet.
In these conventional production processes, however, the pressure reduction effect by the use of the organic assistant such as fatty acid or the like is not sufficient; therefore, the above-mentioned problem is not solved completely, or, even if the pressure reduction effect is sufficient, the density and dispersion of ceramic particles are not sufficient; as a result, there have been problems that the extrudate obtained has low shape retainability and the ceramic structure obtained has defects such as cell breakage, cracking and the like.
Further in the conventional production processes, a relatively long time is required in cutting the extrudate of required external shape (e.g. honeycomb shape) without damaging the shape; thus, there has been a problem also in productivity.
Furthermore in the conventional production processes, there was no systematic index for selection of the organic assistant; therefore, it has been necessary to select an appropriate organic assistant from a substantially limitless number of organic compounds by actually conducting trial productions.
In view of the above problems, the first object of the present invention is to provide a process for production of a ceramic structure, which gives a small load to the production equipment used, which can produce a ceramic structure at high shapability at a high productivity, and which is suitable particularly for production of a honeycomb ceramic structure of small partition wall thickness using a continuous extruder. The second object of the present invention is to provide a process for production of a ceramic structure, which can produce a ceramic structure at high shapability at a high efficiency.
The present inventor made a study in order to achieve the above objects. As a result, the present inventor found out that the first object could be achieved by using a new index, i.e. a haze of a 5 weight % aqueous dispersion or a reduction in peak torque and by selecting an organic assistant having such an index of particular level; and the finding has led to the completion of the present invention. The present inventor also found out that the second object could be achieved by extruding a readily formable bullet at a timing when the torque change (kgfxc2x7m/min) per unit kneading time, of a raw material batch becomes nearly 0 (zero); and the finding has led to the completion of the present invention.
According to the present invention, there is provided a process for producing a ceramic structure, which comprises adding an organic assistant to a ceramic raw material mixture, kneading them to obtain a readily formable bullet, and extruding the readily formable bullet to obtain a ceramic structure, wherein the organic assistant is composed mainly of an organic compound giving a haze of 10 to 90% when made into a 5 weight % aqueous dispersion (hereinafter, this process is referred to as xe2x80x9cthe first production processxe2x80x9d, in some cases).
In the first production process, there can be mentioned, as the organic compound giving a haze of 10 to 90% when made into a 5 weight % aqueous dispersion, one kind of organic compound selected from an ethylene oxide or propylene oxide adduct of one acid selected from the group consisting of caproic acid, caprylic acid and capric acid, an ethylene oxide-propylene oxide adduct of ethylene glycol or glycerine as a base, an ethylene oxide 6 moles adduct of lauric acid, butanol and valeric acid. Further, the total addition amount of the organic assistant is preferably 0.1 to 10 parts by weight, more preferably 0.5 to 2.0 parts by weight per 100 parts by weight of the ceramic raw material mixture, from the standpoints of pressure reduction effect and prevention of inferior shaping.
In the first production process, it is preferred to further add, to the ceramic raw material mixture, an auxiliary agent composed mainly of at least one kind of fatty acid salt or alkylsulfate salt both having a solubility in 25xc2x0 C. water, of at least 1 g per 100 g of water. In this case, the fatty acid salt having a solubility in 25xc2x0 C. water, of at least 1 g per 100 g of water is preferably at least one kind of compound selected from the group consisting of sodium caprate, potassium caprate, magnesium caprate, sodium laurate, potassium laurate and magnesium laurate; and the alkylsulfate salt having a solubility in 25xc2x0 C. water, of at least 1 g per 100 g of water is preferably sodium laurylsulfate. Further, the total addition amount of the auxiliary agent is preferably 0.1 to 2.0 parts by weight per 100 parts by weight of the ceramic raw material mixture, from the standpoints of pressure reduction effect and prevention of inferior shaping.
Also in the first production process, the ceramic raw material mixture is preferably composed of water, a water-soluble cellulose derivative and a granular ceramic raw material.
Further, the first production process has superior features, for example, the process can effectively reduce the heat incessantly generated in the raw material mixture; therefore, the process can be carried out preferably by feeding, into a continuous extruder, a mixture of the ceramic raw material mixture, the organic assistant and the auxiliary agent added as necessary, conducting transfer and kneading of the mixture simultaneously in the extruder, and extruding the resulting readily formable bullet continuously from the die of the extruder to obtain a ceramic structure.
According to the present invention, there is also provided a process for producing a ceramic structure, which comprises adding an organic assistant and as necessary an auxiliary agent to a ceramic raw material mixture, kneading them to obtain a readily formable bullet, and extruding the readily formable bullet to obtain a ceramic structure, wherein the composition and the addition amount of the organic assistant or the auxiliary agent are selected so that the reduction (xcex94T) in peak torque during kneading, represented by the following general formula (1) becomes 20% or more (hereinafter, this process is referred to as xe2x80x9cthe second production processxe2x80x9d in some cases).
(xcex94T)=(Tbxe2x88x92Ta)/Tbxc3x97100xe2x80x83xe2x80x83(1) 
[In the general formula (1), Ta is a peak torque during kneading when the ceramic raw material mixture, the organic assistant and, as necessary, the auxiliary agent have been used; and Tb is a peak torque during kneading when neither organic assistant nor auxiliary agent has been used and only the ceramic raw material mixture has been used.]
In the second production process, the organic assistant is preferably composed mainly of the above-mentioned organic compound giving a haze of 10 to 90% when made into a 5 weight % aqueous dispersion, and is more preferably used in combination with the above-mentioned auxiliary agent composed mainly of at least one kind of fatty acid salt or alkylsulfate salt both having a solubility in 25xc2x0 C. water, of at least 1 g per 100 g of water.
According to the present invention, there is also provided a process for producing a ceramic structure, which comprises feeding, into an extruder, a mixture of a ceramic raw material mixture, an organic assistant and an auxiliary agent added as necessary, conducting transfer and kneading of the mixture simultaneously in the extruder to obtain a readily formable bullet, and extruding the readily formable bullet continuously from the die of the extruder to obtain a ceramic structure, wherein the extrusion of the readily formable bullet is made at a timing when the raw material batch gives a torque change per unit kneading time, of 0.02 Kgfxc2x7m/min or less (hereinafter, this process is referred to as xe2x80x9cthe third production processxe2x80x9d, in some cases).
In the third production process, the organic assistant is preferably composed mainly of the above-mentioned organic compound giving a haze of 10 to 90% when made into a 5 weight % aqueous dispersion, and is more preferably used in combination with the above-mentioned auxiliary agent composed mainly of at least one kind of fatty acid salt or alkylsulfate salt both having a solubility in 25xc2x0 C. water, of at least 1 g per 100 g of water. Further, it is preferred that the composition and the addition amount of the organic assistant or the auxiliary agent are selected so that the above-mentioned reduction (xcex94T) in peak torque during kneading becomes 20% or more.